An injection valve of this type is already known from EP 0 869 278 A1.
High-pressure accumulator injection systems are increasingly being used for supplying fuel to internal combustion engines. Such injection systems are known as common-rail systems (for diesel engines) and HPDI injection systems (for petrol engines). With these injection systems the fuel is delivered with a high pressure pump into a pressure store common to all cylinders of the engine from which the injection valves on the individual cylinders are supplied with fuel. The opening and closing of the injection valves can be controlled electromagnetically or electrically; in the present case electrical piezo actuators are employed to do this.
The extension which occurs in the axial direction on activation of the piezo actuator is exploited via a direct or indirect effective connection with the injection needle of the valve to control the injection of fuel, with a relatively sensitive adjustment between piezo actuator and injection valve being a requirement. The different coefficients of thermal expansion of the piezo ceramic and the surrounding materials produce the problem of compensating for the different length extensions induced by the necessarily wide range of operating temperatures in a motor vehicle in order to avoid a de-adjustment of the valve stroke.
As well as the earlier usual hydraulic compensation element, different non-hydraulic measures to compensate for the length extension of the piezo actuator and of a surrounding actuator housing or valve housing have become known in the interim. For example a piezo actuator valve is known from DE 195 38 791 C2 in which the valve housing itself is embodied as a two-part sleeve with sleeve parts arranged axially behind each other consisting of different materials with different coefficients of expansion. In the generic patent application EP 0 869 278 A1 two further compensation options which are also independent of each other but can also be combined are specified. On the one hand it is proposed that a coefficient of expansion be selected for the material of the actuator housing surrounding the actuator which is almost equal to the coefficient of expansion of the piezo actuator. On the other hand at least one compensation spacer arranged between the piezo actuator and the cover plate of the actuator housing with a relatively high coefficient of thermal expansion is proposed which is suitable for compensating for the small coefficients of expansion of the piezo actuator in relation to the actuator housing.
A particular problem of the temperature compensation described occurs in connection with the necessary pre-stressing of the piezo actuator in the actuator housing. Since tension stresses in the piezo ceramic actuator material are to be avoided at all costs the unpowered piezo actuator is pre-stressed by means of spring force in a defined manner. The powered piezo actuator must thus expand against this pre-stressing. Typically, to create this pre-stressing, as described in the generic EP 0 869 278 A1, a spring arranged between the base plate of the actuator housing and the assigned face side of the piezo actuator is used which presses the actuator against the top plate of the actuator housing and thereby pre-stresses it. Also described is the option of incorporating this type of pre-stressing spring or an assigned control element into the compensation of the different length extension using the choice of material.
Injectors of also known however in which the piezo actuator is pre-stressed within a tubular spring made of steel, which on the one side is welded to the top plate and opposite this, under pre-stressing, welded to the bottom plate. The actuator unit formed from this “housing” together with the piezo actuator accommodated within it under pre-stressing is permanently connected to a valve housing or is built into an injector body. The known measures for compensating for the different length extensions cannot be simply applied to this construction. On the other hand a compensation between piezo actuator and “tubular spring housing” must be undertaken, since otherwise a temperature-dependent change of the pre-stressing and thereby an undesired temperature-dependent activation behavior of the injection valve would be produced.